A variety of circuit topologies have been developed using solid-state switches for conversion of AC power at one frequency to AC power at another frequency. Among the many circuit designs are conventional AC to DC to AC converters, in which the AC power is rectified to a DC voltage applied across DC bus lines and the DC voltage is then converted to AC by an inverter, and matrix converters, in which the input AC power is not rectified but is directly converted using a matrix of bidirectional switching elements (conventionally formed of pairs of transistors). The main advantages of matrix converters are adjustable power factor (including unity), bi-directional power flow, high quality power output waveforms, and the possibility of a more compact product because a large energy storage component (such as a DC bus capacitor) is not needed. However, the matrix converter has not been widely adopted. One reason is that the conventional modulation algorithm for such converters requires an involved and difficult pulse width modulation (PWM) switching strategy. A complicated commutation scheme and an elaborate multi-diode clamp circuit typically must be used for safe operation. See P. Nielsen, et al., “New Protection Issues of the Matrix Converter: Design Considerations for Adjustable Speed Drives,” IEEE Trans. on Industry Applications, Vol. 35, No. 5, 1999, pp. 1150–1161.
A relatively new converter topology is the dual bridge matrix converter. See, L. Wei, et al., “A Novel Matrix Converter with Simple Commutation,” Proceedings of 36th IEEE Industry Applications Society Conference (IAS '2001), Chicago, Ill., USA, 2001, Vol. 3, pp. 1749–1754. The reason this topology is also referred to as a matrix converter is that it shows the same input/output performance as conventional matrix converters, and can also be described by switching matrices similar to the conventional matrix converter. The dual bridge matrix converter also has many of the advantages of the conventional matrix converter, including near sinusoidal input/output waveforms, adjustable input power factor, and a compact physical package because no large energy storage components are required. The dual bridge matrix converter has several advantages over the conventional matrix converter, including reduced difficulty of commutation since all line-side switches turn on and off at zero current and all load-side switches commutate similarly to a conventional DC/AC inverter, and the number of switches required can be reduced under certain constraints. A nine-switch dual bridge matrix converter has been developed that has the least number of switches while still providing high quality input and output waveforms. Three switches are utilized on the input side and six switches are utilized on the output side for three-phase operation. However, a disadvantage of this converter configuration is that its DC link current must be non-negative to guarantee safe operation. If the DC link current becomes negative, some high voltage spikes can be generated because there are no reverse current paths in the line-side converter, and the converter may be damaged by these spikes. It has been suggested that the output power factor should always be higher than 0.866 to guarantee safe operation of the converter. See J. W. Kolar, et al., “Novel Three-Phase AC/DC/AC Sparse Matrix Converter,” Proceedings of 17th IEEE Applied Power Electronics Conference and Exposition, APEC 2002, Vol. 2, 2002, pp. 777–791, and L. Wei, et al., “Matrix Converter with Reduced Number of Switches,” Proceedings of IEEE Power Electronics Specialists Conference, PESC '02, 2002, pp. 57–63.